The present invention relates to an image reduction system in which binary level images are reduced.
In a facsimile apparatus which is a typical example of a conventional static image communication apparatus, a system is adopted in which images are sequentially scanned in a raster direction, encoded and transmitted. Since every pixel of the image must be encoded and transmitted in this system to send the entire image, transmission takes a long time, and therefore the application of the system to image communication services such as an image database service, videotex or the like is difficult.
In order to transmit an overall image quickly, hierarchy coding has been devised. An example of conventional hierarchy coding is shown in FIG. 10. Shown in the figure are frame memories 101 to 104 for storing 1, 1/2, 1/4, and 1/8 reduction images, reduction sections 105 to 107 for creating respectively 1/2, 1/4 and 1/8 reduction images, and encoders 108 to 111 for respectively encoding 1/8, 1/4, 1/2 and 1 reduction images.
The reduction section 105 reduces images from the frame memory 101 by a technique in which the images are subsampled to 1/2 in both the main scanning and subscanning directions to create 1/2-size images and stored in a frame memory 102. Further, the 1/2-size images are reduced by the reduction section 106 to create 1/4-size images and these images are stored in the frame memory 103. Likewise, 1/8-size low-resolution images are created and stored in the frame memory 104.
The images are in turn transmitted in codes beginning with lower resolution images so as to give a rough overall image. The example of FIG. 10(a) shows a case where images are reduced to 1/2, 1/4 and 1/8 in both the main scanning and subscanning directions and encoding is performed in the order of 1/8, 1/4, 1/2 and 1 (1 is the actual size of the image). The images are transmitted in this order. To encode a 1/8 image, the image stored in the frame memory 104 is in turn scanned and entropy encoding such as arithmetic coding is performed by referencing an object pixel to be encoded and the surrounding pixels. As regards a 1/4 image, encoding is performed by the encoder 109 by referencing the pixels surrounding an object pixel from the frame memory 103 and the surrounding pixels of the 1/8 image from the frame memory 104 so that encoding efficiency is improved. Likewise, as regards the 1/2 image of the frame memory 102, the 1/4 image of the frame memory 103 is referenced and encoded by the encoder 110, and as regards the actual-size image of the frame memory 101, the 1/2 image of the frame memory 102 is referenced and encoded by an encoder 111.
Reduction of binary level images is also performed in other than a still image communication apparatus. For example, it includes a case where images are output to printers of different output resolutions from the same image database. Where binary level images read at 400 dpi are output to a printer of 300 or 200 dpi, the images must be reduced to 3/4 or 1/2 respectively in both the vertical and horizontal directions.
Where such reduction is performed, conventionally, subsampling which thins out images at a fixed interval, or a technique wherein images are made to pass through a low-pass filter before they are binarized again, is adopted.
In the hierarchy coding system, as mentioned above, rapid transmission of all images is made possible by transmitting reduced images in codes in an order from a low resolution. Therefore, it is necessary to leave information in reduced low-resolution images so that the full image can be easily scanned.
A conventional reduction method as described above has a drawback in that information is lost. FIG. 11A shows an example in which pixels with x marks in the original image (1) are subsampled to obtain an image (2) reduced to 1/2 in the vertical and in the horizontal directions.
In the case of subsampling only, if one line L lies in the middle of subsampling points (the x marks in the FIG. 11A), this line is lost during the reduction. To eliminate this drawback, a technique has been devised in which subsampling is performed after filtering is performed. An example of this is shown in FIGS. 11B and 12. The x marks in FIG. 11B denote sampling points. In the example of FIG. 11B, before subsampling is performed, a low-pass filter having the coefficients shown in FIG. 12 is applied to convert filter output to binary values. For example, when filter input is 8 or greater, filter output can be defined as 1; when smaller than 8, 0. However, even a system using filtering cannot remove the drawback that the line is lost when a vertical line L2 in the original image in the example of FIG. 11B lies between portions where subsampling is performed.
Accordingly, unless a line of the width of one pixel is preserved in any system in which reductions are repeated many times, the line will ultimately be lost in a low-resolution image. Hence, a narrow line such as a one-pixel line must be preserved irrespective of sampling points.
With a binary level pseudo-half-tone image such as a dither image or the like a drawback exists in that such a reduction may cause density information to be lost depending on sampling points. Where dots are scattered by a pseudo-half-tone process, in particular, in a low density or a high density, a problem exists in that half-tone is abruptly lost or density is reversed.